Viewport background images disappeared when .blend file closed and opened
Some things to check for that would impact the visibility of your background images, besides being in the proper type of view (orthographic, as distinguished from perspective view) include the following.

If you used the background images panel in the properties shelf of the 3D viewport, are the background image files still on your computer in the same place as they were when you loaded them? 

If the images are in the same location, is the background images panel activated in the new instance of Blender into which you loaded your previously saved file? 

Are you looking at the right view in the 3D viewport? An image is only visible if Blender thinks it is looking at the same view, so that if you used the facility to rotate the image in any direction from one of the six background image views, even if you rotate it back to the original view, Blender will still think it's in a user view, and the image will not be visible. Try pressing the numpad keys for the view for which you have background images located: for example, if you have a top view, press the  Numpad-7 key; a left view, press CTRL-Numpad-3 .

It might expedite getting an answer if you upload your blend file to blend-exchange.giantcowfilms.com, and edit your question to provide a link to the blend file.